Marines are taken in, alas, by possibly false ones (6)
I believe the answer is:
alarms
'possibly false ones' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how one could define the other.
'marines are taken in alas' is the wordplay.
'marines' becomes 'RM' (abbreviation for 'Royal Marines').
'are taken in' is an insertion indicator.
'rm' placed inside 'alas' is 'ALARMS'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for alarms that I've seen before include "Frightens, makes afraid" , "Warning clocks that wake us up" , "Frightens or alerts" , "Fills with apprehension" , "Warning sounds" .)